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We introduce multi-branch repeater laser stations (MLSs) for the dissemination of an ultra-stable signal from one point to multiple users and the simultaneous evaluation of the stability and accuracy of multiple links. We perform the study of the noise floor of this new instrument. We present then an optical fiber network of 4800 km built with three MLSs and 13 repeater laser stations (RLSs). We show the multi-user optical frequency dissemination on four links totalizing 2198 km with uncertainties below 1.1 × 10 −19 . The robustness of the network over two years is presented and stability and accuracy at 10 7 seconds integration time are finally showed.

Variations in optical fiber length and refractive index are induced by environmental perturbation, resulting in an additional dynamic propagation delay in fiber-based time synchronization systems, which deteriorate their transfer stability. This disadvantage can be significantly reduced by transmitting the time signal in both directions through fiber and constructing a feedback loop to compensate the propagation delay at the remote end of the link. This paper proposes an analog-digital hybrid proportional integral derivative (PID) control compensation system based on the time-frequency phase-locked loop (TF-PLL). The system is designed to keep the merits of wide servo bandwidth, servo accuracy, and a large dynamic delay compensation range up to 1 s, which is much greater than that reported in previous studies. For proving the validity of this proposed scheme, a self-developed optical fiber time synchronization equipment based on the delay compensation system is applied. The delay compensation system is used on a 1100-km long laboratory optical fiber, and the results show that the time synchronization stability in terms of time deviation (TDEV) is less than 5.92 ps/1 s and 2.56 ps/10,000 s. After successful laboratory evaluation, the proposed system is installed on a real 988.48-km line between the Xi’an Lintong branch of the National Time Service Center (NTSC) and Linfen City, Shanxi Province, realizing the time synchronization of 10 stations along the optical fiber link. The experimental results in the 988.48-km link illustrate that the measured time difference with a peak-to-peak value of 176 ps, the standard deviation of 19.3 ps, and a TDEV of less than 10.49 ps/1 s and 2.31 ps/40,000 s is achieved. The high-precision time delay compensation system proposed in this paper is simple, reliable, and accurate; has a wide range of compensation; and opens up a feasible scheme for providing synchronized time signals to multiple users over the long-distance field optical fiber networks.

Two methods for compensation of polarization fading in microwave photonic links have been studied. Comparison of noise characteristics has been performed and factors limiting the dynamic range of microwave photonic link have been analyzed on an example of a link with an external remote electrooptic modulator. The possibility of reaching spurious-free dynamic range close to the shot-noise limit SFDR 3 ≈ 116 dB Hz –2/3 has been demonstrated for 1000 m microwave photonic link.

We propose and experimentally demonstrate a tunable frequency-doubling optoelectronic oscillator (FD-OEO) based on a single-bandpass dispersion-induced microwave photonic filter (MPF) consisting of a Mach–Zehnder modulator (MZM), a linearly chirped fiber Bragg grating and polarization-multiplexed dual-loop. Thanks to the polarization dependence of the MZM, a special double sideband modulation is implemented where the optical carrier (OC) and subcarriers are orthogonally polarized. By simply tuning the PC in the OEO loop, the phase difference between the orthogonal polarization carrier and two sidebands can be controlled, and thus the center frequency of the fundamental OEO can be tuned. Furthermore, a PC and a polarizer are placed outside the OEO to achieve optical carrier suppression (OCS) modulation, which ensures that a frequency-tunable microwave signal at the second-harmonic frequency is generated. In the experiment, a fundamental frequency signal with tunable frequency from 3.6 to 6.85 GHz and FD-OEO with a tunable frequency range from 7.2 to 13.7 GHz are generated.

Efficient simultaneous transmission of light with a power of more than 2 W at a wavelength of 976 nm and an optical carrier for transmitting a high-frequency analog signal at a wavelength of 1550 nm over a distance of 1 km over a standard single-mode fiber was experimentally demonstrated. Electrical power up to 350 mW (5 V, 70 mA) was obtained from a multi-junction silicon photocell, resulting in the optical transmission efficiency of about 70% and a photocell efficiency of 25%. The power transmission did not affect the transmission of the high frequency analog signal. Key broadband analog transmission characteristics such as noise figure (NF < 25 dB) and spurious-free dynamic range (SFDR3 > 117 dB/Hz2/3) were achieved and were close to the fundamental shot noise limit. This approach is promising for powering a remote antenna unit in optical fronthaul architecture.

The fibre-optic microwave photonic link has become one of the basic building blocks for microwave photonics. Increasing the optical power at the receiver is the best way to improve all link performance metrics including gain, noise figure and dynamic range. Even though lasers can produce and photodetectors can receive optical powers on the order of a Watt or more, the power-handling capability of optical fibres is orders-of-magnitude lower. In this paper, we propose and demonstrate the use of few-mode fibres to bridge this power-handling gap, exploiting their unique features of small acousto-optic effective area, large effective areas of optical modes, as well as orthogonality and walk-off among spatial modes. Using specially designed few-mode fibres, we demonstrate order-of-magnitude improvements in link performances for single-channel and multiplexed transmission. This work represents the first step in few-mode microwave photonics. The spatial degrees of freedom can also offer other functionalities such as large, tunable delays based on modal dispersion and wavelength-independent lossless signal combining, which are indispensable in microwave photonics. Microwave photonics: few-mode communication The use of few-mode, large-core optical fibres can dramatically enhance the performance of microwave photonics communications. Fibre-optic microwave photonic links have become a basic building block for microwave photonics, but their performance is currently limited by the power-handling capability of optical fibres. Now, Guifang Li at the University of Central Florida in the USA and co-workers reduced nonlinear crosstalk in a wavelength-division multiplexed communications link by 30 decibels through employing a 20-kilometre-long few-mode fibre. Using a few-mode fibre allowed them to overcome the optical nonlinearity problems that tend to plague single-mode fibres when large optical signal powers are used. Experiments using single-wavelength channels also showed improved transmission. Future research will explore using few-mode fibres to perform functions such as tunable delay and wavelength-independent lossless signal combination.

In this article, we review the latest progress on data center interconnect (DCI). We then discuss different perspectives on the 400G pluggable module, including form factor, architecture, digital signal processing (DSP), and module power consumption, following 400G pluggable optics in DCI applications. Next, we experimentally investigate the capacity-reach matrix for high-baud-rate and high-order quadrature amplitude modulation (QAM) single-carrier signals in the unamplified single-mode optical fiber (SMF) link. We show that the 64 GBd 16-QAM, and 64-QAM signals can potentially enable 400 Gb/s and 600 Gb/s DCI application for 40 km and beyond of unamplified fiber link.

As the demand for high data volumes keeps increasing in optical access networks, transmission capacities and distance are becoming bottlenecks for passive optical networks (PONs). To solve this problem, a novel scheme based on multi-twin single sideband (SSB) modulation with direct detection is proposed and investigated in this paper. At the central office, two SSB signals are generated simultaneously with the same digital-to-analog converters (DACs). The twin-SSB signal is not only robust against frequency selected power fading introduced by chromatic dispersion (CD), but also improves the spectral efficiency (SE). By combining a twin-SSB technique with multi-band carrier-less amplitude/phase modulation (multi-CAP), different optical network units (ONUs) can be supported by flexible multi-band allocation based on software-reconfigurable optical transceivers. The Kramers–Kronig (KK) scheme is adopted on the ONU side to effectively mitigate the signal–signal beat interference (SSBI) induced by the square-law detection. The proposed system is extensively studied and validated with four sub-bands using 50 Gbps 16 quadrature amplitude modulation (QAM) modulation for each sub-band using numerical simulations. Digital pre-equalization is introduced at the transmitter-side to balance the performance of different ONUs. After system optimization, a bit error rate (BER) threshold for hard decision forward error correction (HD-FEC) code with 7% redundancy ratio (BER = 3.8 × 10−3) can be reached for all ONUs over 50-km standard single-mode fiber.

An analytic design-oriented model of microwave optical links has been developed. The core of the model is the non-linear and noise model of a Mach-Zehnder LiNbO interferometer. Both a 100 MHz–20 GHz link and a linearized microwave link, comprising an auxiliary modulator, have been designed and prototyped by using the model.

We present a tapped tunable delay line filter for radiofrequency (RF) photonic filtering applications, capable of rapid tunability over a wide RF bandwidth limited only by the optical components’ losses, while maintaining independence from polarization state. Multiple fiber taps with contrasting dispersion slopes are used in intensity-modulated direct detection microwave photonic links. A temporal delay is generated between the signals within each arm of the link. Once a signal is received using balanced differential detection, nulls are generated as a function of the laser's operating wavelength. Tuning of the laser allows for a rapid shifting of the nulls in the RF spectrum to dynamically mitigate co-site interference. Through this method we demonstrate the potential for rapid tunability over the RF spectrum by the variation of the operating wavelength of the optical carrier.